1. Field of the Invention
The present invention relates generally to a method of forming a resist pattern. More particularly, the invention relates to a method of forming a resist pattern or a patterned resist layer using its reflow phenomenon or behavior, which is preferably used in a lithography process for forming a minute shape or profile in semiconductor device fabrication.
2. Description of the Related Art
In recent years, the trend to increase the integration scale and the performance of semiconductor devices, i.e., Large-Scale Integrated circuit devices (LSIs), has been progressing rapidly. According to this trend, how to suppress the rising fabrication cost has become an important problem to be solved.
In response, an improved method of forming a pattern was developed and disclosed so far, which is capable of reducing the count of necessary processes to thereby lower the fabrication cost. In this improved method, a first etching process is carried out using a resist pattern as a mask. Then, the resist pattern is modified by heating due to its “reflow” phenomenon, in other words, the pattern is heated in such a way as to soften and reflow, resulting in a modification of the pattern. Subsequently, a second etching process is carried out using the resist pattern thus modified as a mask, thereby forming a desired shape or profile. This method, which is termed the “resist reflow” method, has an advantage that the count of necessary processes is reduced. This is because the first and second etching processes can be carried out with substantially the same resist pattern, in other words, a single lithography process is necessary for conducting the first and second etching processes.
FIGS. 1A and 1B show the concept of the prior-art resist reflow method described above. FIGS. 1A and 1B are schematic cross-sectional views before and after the reflowing process, respectively.
First, as shown in FIG. 1A, a resist pattern (i.e., a patterned resist layer) 101 having a specific shape is formed on a base material or layer 102. The pattern 101 is typically formed by pattering a resist layer made of an appropriate resist material. The pattern 101 is used as a mask in a subsequent first etching process (not explained here).
After the first etching process using the pattern 101 is completed, a solvent is penetrated into the resist pattern 101 by way of its surface by exposing the pattern 101 to a vapor of a soluble solvent or the like. In this stage, the viscosity of the pattern 101 lowers due to penetration or absorption of the solvent and therefore, the pattern 101 softens and reflows. This reflowing behavior of the pattern 101 progresses in a most stable form with respect to energy under the effects of the surface tension, the re-volatilization of the solvent penetrated into the pattern 101, and the wettability of the underlying surface (i.e., the surface of the base material or layer 102). When the pattern 101 is modified to have a desired shape due to the reflowing behavior, the exposure of the pattern 101 to the vapor is stopped and a drying process for drying the solvent penetrated into the pattern 101 is conducted, thereby stopping the modification of the pattern 101. Thus, the reflowing process of the pattern 101 is completed. As a result, as shown in FIG. 1B, a modified resist pattern 101a is formed on the base material or layer 102. This pattern 101a is used as a mask in a subsequent second etching process (not explained here).
The modified resist pattern 101a shown in FIG. 1B is considerably different in shape (i.e., outer form) from the initial resist pattern 101 shown in FIG. 1A. However, by properly adjusting the condition for the reflowing process, the level or degree of modification of the initial pattern 101 is changeable according to the necessity.
FIGS. 2A to 2C show an application example of the prior-art resist reflow method as shown in FIGS. 1A and 1B, in which the method is applied to the fabrication of a Field-Effect Transistor (FET). FIG. 2A is a schematic cross-sectional view after the first etching process and before the reflowing process, FIG. 2B is a schematic cross-sectional view after the reflowing process, and FIG. 2C is a schematic cross-sectional view after the second etching process.
As shown in FIG. 2A, a single-crystal or polycrystalline silicon (Si) layer 112 is formed on a dielectric base material or a dielectric layer 110. A source electrode 114a and a drain electrode 114b are formed on the Si layer 112 to be apart from each other at a distance, forming a space S′ between the electrodes 114a and 114b. These electrodes 114a and 114b are formed by patterning a chromium (Cr) layer.
An initial resist pattern or a patterned resist layer 111 is formed on the source and drain electrodes 114a and 114b. This pattern 111 is obtained by narrowing the width of its original resist pattern (not shown) used for forming the electrodes 114a and 114b. This narrowing process is conducted by a known method and thus, no explanation is presented here. The initial pattern 111 has an opening to be overlapped with the underlying space S′.
A reflowing process of the initial resist pattern 111 is carried out from the state of FIG. 2A. This reflowing process is realized by penetrating a solvent into the resist pattern 111 by way of its surface by exposing the pattern 111 to a vapor of a soluble solvent or the like. In this process, the viscosity of the initial pattern 111 lowers due to penetration of the solvent and therefore, the pattern 111 softens and reflows gradually in a most stable form with respect to energy under the effects of the surface tension, the re-volatilization of the solvent penetrated into the pattern 111, and the wettability of the underlying surface (i.e., the surface of the underlying Si layer 112). As a result, the shape of the pattern 111 is modified to expand horizontally.
When the initial resist pattern 111 is modified to have a desired shape due to the reflowing behavior, the exposure of the pattern 111 to the vapor is stopped and a drying process for drying the solvent penetrated into the pattern 111 is conducted, thereby stopping the modification of the pattern 111. Thus, the reflowing process is completed. As a result, as shown in FIG. 2B, a modified resist pattern 111a is formed on the Si layer 112.
As seen from FIG. 2B, the modified resist pattern 111a is expanded horizontally in such a way as to contact the surface of the Si layer 112 and to entirely cover the source and drain electrodes 114a and 114b. The pattern 111a has a varying thickness according to the profile on the surface of the layer 112. In other words, the pattern 111a has a less thickness uniformity. The depressed part A of the pattern 111a, which is located between the electrodes 114a and 114b, has a relatively smaller thickness.
Following this, a subsequent second etching process for the underlying Si layer 112 is carried out by dry etching using the modified resist pattern 111a as a mask. Thus, the Si layer 112 is selectively etched, thereby forming a Si island 112a on the surface of the base material or layer 110. At this time, the source and drain electrodes 114a and 114b are located on the Si island 112a. The state at this stage is shown in FIG. 2C.
With the above-described prior-art method of forming a resist pattern shown in FIGS. 2A to 2C, however, the surface of the initial resist pattern 111 continues to absorb the solvent and soften during the reflowing process. Moreover, as the reflowing behavior of the pattern 111 advances, the vapor of the solvent to be adhered to the underlying surfaces of the pattern 111 will increase and as a result, the wettability of these surfaces to the pattern 111 will improve or rise. In this way, the more the reflowing behavior advances, the higher the reflowing rate of the pattern 111 rapidly. Accordingly, there is a disadvantage that accurate formation of the modified resist pattern 111a as desired is difficult and that the in-plane uniformity of the quality of the modified pattern 111a is lowered.
Taking the said disadvantage into consideration, to raise or improve the accuracy and uniformity of the modified resist pattern 111a, it is necessary for the above-described prior-art method shown in FIGS. 2A to 2C to complete or finish the reflowing process in such a way that the degree or level of modification of the initial pattern 111 does not exceed a certain value. This means that the initial resist pattern 111 is unable to be modified to a pattern widely different from the said pattern 111. In addition, since only the surfaces of the pattern 111 dissolves in an early stage of the reflowing process, the extended parts of the modified pattern 111a will be thinner than the remaining part thereof. Thus, the extended parts will have an insufficient resistance property against dry etching used in the second etching process.
On the other hand, another method using a bank or embankment can be thought to raise the accuracy of the modified resist pattern 111a. In this method, a bank or embankment is formed in the preceding patterning process of the underlying layer or material to the reflowing process in such a way as to block the excessive extension of the dissolving or softening pattern 111 in the subsequent reflowing process. However, this method is not preferred, because the shape or pattern of the underlying layer or material is restricted by the formation of the bank or embankment.